The fact that the frequency band that is allocated for wireless access systems is limited by nature dictates several restrictions that are imposed on the network operators, which may require the equipment manufacturers to provide creative solutions for exploiting the limited available licensed spectrum more efficiently. An example for such a solution may be found in WO 2006048873 that describes combining operations carried out according to an old technology together with operations under a new technology, instead of simply dividing the licensed frequency band into two sub-bands of frequencies, each allocated for transmissions compatible with one of the two technologies. By combining two technologies that utilize different PHY modes, the solution provided by WO 2006048873 enables smoothing the transition from one generation to another while providing an efficient solution to the utilization of the frequency band.
But shifting from one technology to another is not the sole reason for problems associated with the usage of a frequency band, due to the ever growing traffic and number of users that utilize a given frequency band. As a result, the cells become smaller and the interference problems more acute.
U.S. Pat. No. 7,177,645 describes a method for allocation of time-sub-frames for operating at frequencies that are subjected to interferences. The time frame is divided into two time sub-frames and the wireless links along which communications will be transmitted in each of these sub-frames are selected in such a way that during each sub-frame, none of the operative links will be subjected to interferences caused by communications transmitted during that sub-frame along any of the other links. Also, the time frames allocated for communication along the channels that experience interferences are divided into two time sub-frames, where one of the time sub-frames is reserved for communicating signals to/from subscriber terminals that are not subjected to interferences, while another time sub-frame is reserved for communicating radio signals to/from terminals that are subjected to interference. Time-Division Duplex (TDD) is the application of time-division multiplexing to separate outward and return signals. It emulates full duplex communication over a half duplex communication link. As the amount of uplink data increases, more bandwidth can dynamically be allocated to that and as it shrinks it can be taken away. Problems associated with coexistence of adjacent TDD networks are resolved by synchronizing the transmit and the receive intervals of the Base Stations, thereby eliminating both the BS-BS and SS-SS interference.
A special situation occurs in case of FDD/TDD collocated networks.
Frequency-division duplexing (FDD) technology means that the BS (Base Station) transmitter and the SS (subscriber station) transmitter operate at different carrier frequencies. The radio unit can work either in a full-duplex mode (transmitting and receiving in the same time) or in a half-duplex mode (transmitting and receiving at different times). In general, the Base Stations operates using a full duplex mode, while the subscriber operates in either a full-duplex mode or a half-duplex mode. The filters used in FDD full-duplex radios reject the transmitted frequency from the received signal, so that frequency separation resolves the BS-BS and SS-SS coexistence problems. The frequency separation is high enough to allow the realization of such radio filters.
Now, if a TDD based wireless network and an FDD based wireless network should coexist with each other, there might be severe interference problems due to the limited frequency separation, to the fact that the frequency gap between the operational frequencies of the two systems may be too small for realizing strong radio filters or due to roaming requirements for MSs, which should operate in any country over the whole available frequency band having a flexible partition between FDD and TDD operational frequencies, which do not allow to separate the FDD bands from the TDD bands by radio filters. Section 8.3 of the IEEE 802.16m System Requirements Document (IEEE 802.16m-07/002r4) emphasizes coexistence scenarios for a large number of various situations and technologies, including sharing of the same-licensed frequency band and coexistence with devices operating in adjacent LE (“license-exempt”) bands. The interference problems associated with neighboring networks using the same band of frequencies are intensified when the networks are collocated. Collocated networks are networks that are deployed within the same geographical area, thus having a higher probability of interfering with each other.
In general, there are several scenarios in which coexistence of collocated systems might cause interference:
1. SS to SS Interference
Experienced for example in cases of small (e.g. 1 m) separation distance between two adjacent subscribers (referred to herein and throughout the specification as “SSs” or “MSs”), when one SS transmits communications within a time interval during which the other SS is in a state of receiving communications. In such a case, there is a high probability, especially in the case of FFD/TTD collocated systems, that harmful interference are experienced by the receiving SS. This is the most problematic case, because the levels of out-of-channel emissions are high and the RF (radio) filters may be inefficient for radios operating at adjacent frequency bands or non-existing for radios operating in adjacent frequency channels. Additionally, the radio front-end could be saturated. The SS-SS interference can be also experienced in cases where the separation distance between the two collocated radios is significantly higher than 1 m mentioned above.
2. SS to BS/Relay Interference
Experienced by BS or Relay in cases where the distance between a foreign SS and a BS is relatively small.
3. BS/Relay to BS/Relay Interference
Experienced in cases of collocated TDD-FDD wireless networks (e.g. WCDMA and 802.16m or LTE and 802.16) or un-synchronized TDD (i.e. LTE and 802.16m).
4. BS/Relay to SS Interference
Experienced by SS when the distance between the SS (being a foreign SS) and a BS are relatively small.
Thus, there is a need for a method for reducing the interference occurring in collocated networks operating in compliance with different technologies and different spectrum access methods.